1. Field of the Invention
The present invention relates to the processing of an echo between two transmission channels with a coupling between them.
2. State of the Prior Art
In particular, the invention is destined to suppress acoustic echo in a telephone set or other telephone terminal of the hands-free type. As shown in FIG. 1, the set has one or several loudspeakers HP as well as one or several microphones MI, all fixed. The acoustic echo is the signal picked up by the microphone(s) of the set coming from the loudspeaker(s) by acoustic coupling. This coupling can be due to solid-state or aerial transmission; in the latter case, the environment of the terminal plays a decisive role.
An echo processing device DT is interconnected in the reception channel of the set receiving a received signal RECU from the subscriber telephone line LT and transmitted as a signal DIFFUSE broadcast by loudspeaker(s), and in the emission channel of the set transmitting a microphonic signal picked up by the microphone(s) and transmitted as an emitted signal EMIS in the line LT. The microphonic signal is the sum of the local speech signal coming from the local subscriber and an acoustic echo coming from the loudspeaker(s).
For transmission terminals such as hands-free set and group or teleconference terminals, the acoustic echo can cause considerable inconvenience to the far end speaker, particularly in the case of long transmission periods between the local subscriber and the speaker via the telephone network. The decrease of this echo to a satisfactory level in the emitted signal EMIS constitutes the main purpose of the echo processing device DT.
Conventionally, according to a first known embodiment, a device DT uses gains switching between the channels to considerably weaken the echo. Variable gains are applied to the reception and emission signals to globally ensure a corrective gain which reduces the value of the acoustic coupling. This introduces a hindrance into the conversation caused by the transmission of the signals in the line LT close to the half-duplex mode.
To avoid this, a second type of echo processing device uses the adaptive identification technique which carries out an echo cancellation by subtraction of an echo estimated on the basis of an estimation of an acoustic coupling pattern. The cancellation performances therefore depend directly on the quality of estimation of the acoustic coupling. The nature of the acoustic coupling requires that the pattern take into account a large number of acoustic reverberations caused by the environment of the set.
The device must estimate an important fraction of the impulse response representing the acoustic coupling. The conventional algorithms of adaptive identification do not work for "long" impulse responses of more than ten milliseconds. Local speech is seen as a noise by the algorithm and considerably perturbs identification. The algorithm can also distort local speech which should ideally be transmitted undisturbed. The environment of the set is variable, due e.g. to the movement of the people in the vicinity of the set, and the algorithm must "follow these changes" in order to remain efficient.